Axenic cultures of the N2‐fixing marine cyanobacterium Crocosphaera watsonii exhibit very high B12 excretion rates (up to 40 times higher per cell), compared to the smaller non‐N2‐fixing strain of marine Synechococcus. The effect of N availability on vitamin synthesis is also evident in the non‐diazotrophic strain, as they release five times more B12 under N‐replete conditions relative to N‐limiting conditions, although this value is still an order of magnitude smaller than the amount produced by the diazotroph. The cyanobacterial contribution to the oceanic B12 supply may be ~ 50 times higher than the contribution of heterotrophic bacteria. Oceanic cyanobacteria do not compete for exogenous B12 with other prokaryotes and eukaryotes, but instead are obligate producers, thereby defining a unique dual ecological role for N2‐fixing cyanobacteria in the oligotrophic ocean. They provide both a source of “new” bioavailable nitrogen and the B12 needed to support the growth of larger auxotrophic eukaryotic phytoplankton important for the biological carbon pump.